Photopatterning of thick film conductor compositions has long been used to achieve very fine lines (three mils or less line/space pitch) found in high density circuits. Unfortunately, the thickness or the height of lines that are achievable with photopatterning methods has been rather low. Since the metal powders found in compositions that are used in the photosensitive methods are opaque, the surface must be over-exposed to achieve crosslinking beneath the surface. The overexposure leads to the phenomena of undercut and edge curl. Upon exposure the top surface width of the patterned print is greater than the underlying regions resulting in undercut of the patterned edges during development, and subsequent edge curl when the patterned print is sintered at high temperatures.
In further explanation, edge curl and undercut problems are caused by intense surface polymerization that occurs in photopatterned films that have particulate phases that strongly absorb UV light. Since acrylic monomers shrink during polymerization by several percent or more (as much as 18%), a film that polymerizes only on the top surface will do two things. First, it will cause monomer at the surface to strongly crosslink. Secondly, monomer directly beneath the polymerized layer, migrates to the boundary of the polymerized region, where it reacts with the partially polymerized material at the boundary. Thus, the region directly beneath the polymerized region becomes depleted in monomer, causing the region in the film just beneath the polymerization front to be more rapidly washed out during development. However, both the crosslinked top region and the region of the film below the boundary that has not been depleted in monomer are more slowly washed out. This causes a washed out gap in the edges of patterned features at the point between the polymerization boundary and the deeper, unpolymerized portions of the film. Since the top and bottom regions of the film edge are, in effect, disconnected by this crease or gap, during firing the two regions sinter separately, permitting the top region to pull upward. It is believed that the edge curl turns upward because the surface regions of the film are the most heavily polymerized, and have the heaviest concentration of monomer. Since the organic content of the top layer of the film is higher, it has a lower metal content, and will shrink more during firing than regions with higher metal content below the surface. This creates a situation similar to that in a bimetallic strip or thermostat. One layer shrinks more than the other, and the film curls in that direction, i.e., upward.
If excessive crosslinking can be sufficiently reduced near the surface of the exposed film, in principle, there will be no weak region beneath the exposed film, hence no undercut or edge curl. For example, it has been demonstrated that edge curl is reduced in gold films that have less exposure to UV light, compared to more heavily exposed films. The problem with using less exposure is that finer features are lost, so resolution and edge definition both suffer as exposure is reduced. This invention offers a solution to this problem. Therefore, the present invention reduces or eliminates edge curl while simultaneously increasing the achievable height of the fine lines to over twelve microns after firing and substantially reducing the intensity of exposure to UV light necessary to generate a high quality pattern. This is accomplished with a new composition and process in which it is not necessary to have light penetrate completely through a printed layer because of the use of a thermal assist catalyst in the composition.